Decision-feedback equalizer with maximum-likelihood sequence estimation and associated methods

ABSTRACT

A Decision-Feedback Equalizer (DFE) includes a Maximum Likelihood Sequence Estimator (MLSE) for estimating a symbol sequence. The DFE also includes a signal level decoder, and a delay line having a first plurality of taps being connected to the output of the signal level decoder, and a second plurality of taps being connected to the output of the MLSE. Furthermore, the DFE has an error signal generator having a first input connected to the input of the signal level decoder, and a second input connected to the output of the signal level decoder for adjusting coefficients of the taps. The symbol delay line has been split into two sections with the output of the MLSE being fed to part of the delay line. Because symbols in the delay line are adjusted with the coefficients and fedback to the signal level decoder, the output of the MLSE is able to correct symbols whenever its output is available. In such a symbol updating approach, error propagation in the delay line is avoided during an error event.

This application claims the benefit of provisional application Ser. No.60/151,696 filed Aug. 31, 1999.

FIELD OF THE INVENTION

The present invention relates to digital communications, and moreparticularly, to Decision-Feedback Equalization (DFE) andMaximum-Likelihood-Sequence-Estimation (MLSE) in a digital receiver.

BACKGROUND OF THE INVENTION

Digital communication systems, such as standard telephone twisted pairloops or wireless radio communication systems, are used to convey avariety of information between multiple locations. With digitalcommunications, information is translated into a digital or binary form,referred to as bits, for communication purposes. A pair of binary bitsform a symbol. A transmitter maps the bit stream into a modulated symbolstream, converts the modulated symbol stream to a signal and transmitsthe signal. A digital receiver receives the signal, down converts thesignal to a low frequency signal, samples the low frequency signal andmaps the sampled signal back into an estimate of the information.

The communication environment presents many difficulties that effectcommunications. For example, dispersion occurs, wherein crosstalk orother noise disturbances may give rise to signal errors. To reduce theerrors, it is known that a Maximum-Likelihood-Sequence-Estimation (MLSE)equalizer may be employed. Such an equalizer considers varioushypotheses for the transmitted symbol sequence, and, with a model of thedispersive channel, determines which hypothesis best fits the receiveddata. This can be realized using the Viterbi Algorithm. Thisequalization technique is well-known to those skilled in the art, andcan be found in standard text books such as J. G. Proakis, DigitalCommunications, 2d ed., NY: McGraw-Hill, chapter 6, 1989.

A receiver is described in an article in IEEE Transactions onInformation Theory, January 1973, pages 120-124, F. R. Magee, Jr. and J.G. Proakis: “Adaptive Maximum-Likelihood Sequence estimation for DigitalSignaling in the presence of Intersymbol Interference”. The articledescribes a channel equalizer which includes a viterbi analyzer havingan adaptive filter as a channel estimating circuit. Received symbols arecompared successively with hypothetical symbols and those hypotheticalsymbols which coincide closest with the received symbols are selectedsuccessively to form an estimated symbol sequence. The parameters of theadaptive filter are adjusted successively to the changed channel, withthe aid of the selected, decided symbols. A description of the viterbialgorithm is given in an article by G. David Forney, Jr.: “The ViterbiAlgorithm” in Proceedings of the IEEE, Vol. 61, No. 3, March 1973. Thearticle also describes in some detail the state and state transitions ofthe Viterbi algorithm and also how these state transitions are chosen toobtain the most probable sequence of symbols.

However, the MLSE equalizer is highly complex because, for example, theMLSE equalizer is based upon the assumption that symbol interferenceextends over the entire transmitted message and that the communicationchannel varies with time. Thus, implementation of the MLSE is expensive,requires a lot of hardware and/or software resources, and ispower-consuming. Accordingly, a decision feedback equalizer (DFE) isknown as an alternative to the MLSE. DFE arrangements are advantageousin that they exhibit low computational complexity. U.S. Pat. No.5,353,307 to Lester et al. and other publications disclose adaptiveequalizers for simulcast receivers that employ Lattice-DFE andKalman-DFE techniques.

Hybrid arrangements that combine various equalization techniques havealso been proposed. For example, an article by W. U. Lee and F. S. Hill,Jr.: “A Maximum-Likelihood Sequence Estimator with Decision-FeedbackEqualization,” in IEEE transactions on communications, September 1977,proposes a DFE as a pre-filter which limits the complexity of a MLSEimplemented by the Viterbi algorithm for channels having a long impulseresponse. However, the proposed scheme has a disadvantage of feeding theDFE with slicer output. This may cause error propagation in the delayline and affect the performance of the MLSE.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of theinvention to improve the performance of a decision-feedback equalizer(DFE) by reducing error propagation in the delay line.

This and other objects, features and advantages in accordance with thepresent invention are provided by a DFE including a first summing nodehaving a first input for receiving an input signal, a second input forreceiving a second feedback signal, and an output. A maximum likelihoodsequence estimator (MLSE) for estimating a symbol sequence has an inputconnected to the output of the first summing node, and has an output. Asecond summing node has a first input connected to the output of thefirst summing node, a second input for receiving a first feedbacksignal, and an output. The DFE also includes a signal level decoderhaving an input connected to the output of the second summing node, anda delay line. The delay line includes a first plurality of taps beingconnected to the output of the signal level decoder, and generatingrespective first tap signals based upon respective first coefficients,and a second plurality of taps being connected to the output of theMLSE, and generating respective second tap signals based upon respectivesecond coefficients. Furthermore, the DFE has a first summing circuitfor summing the first tap signals to generate the first feedback signal,a second summing circuit for summing the second tap signals to generatethe second feedback signal, and an error signal generator having a firstinput connected to the input of the signal level decoder, and a secondinput connected to the output of the signal level decoder for adjustingthe first and second coefficients.

The MLSE preferably has a partial output, and the delay line furthercomprises a third tap connected to the partial output of the MLSE. Thepartial output of the MLSE outputs a partially estimated signal basedupon the estimated symbol sequence. The third tap generates a third tapsignal based upon a third coefficient, and the first summing circuit maysum the first and third tap signals to generate the first feedbacksignal. Also, the MLSE preferably estimates the symbol sequence basedupon the M-algorithm.

Objects, features and advantages in accordance with the presentinvention are also provided by a method of estimating symbol sequencesof an input signal comprising a plurality of symbols. The methodincludes summing an input signal and a second feedback signal togenerate a first summed signal, and summing the first summed signal witha first feedback signal to generate a second summed signal. The secondsummed signal is level decoded to generate a decoded signal, andrespective first tap signals are generated from the decoded signal basedupon respective first coefficients. Also, the first tap signals arecombined to form the first feedback signal. Second tap signals aregenerated from a symbol output signal based upon respective secondcoefficients, and the second tap signals are combined to form the secondfeedback signal. A maximum likelihood sequence estimation is performedfor estimating a symbol sequence of the first summed signal to providethe symbol output signal.

Also, an error signal may be generated based upon the second summedsignal and the decoded signal for adjusting the first and secondcoefficients. Furthermore, performing the maximum likelihood sequenceestimation may comprise generating a partial output signal, and a thirdtap signal may be generated from the partial output signal based upon athird coefficient. The partial output signal comprises a partiallyestimated signal based upon the estimated symbol sequence. Here, the sumof the first and third tap signals forms the first feedback signal.Moreover, the maximum likelihood sequence estimation is preferably basedupon the M-algorithm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a DFE in accordance with thepresent invention.

FIG. 2 is a flowchart illustrating the method steps in accordance withthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Referring to FIG. 1, the Decision-Feedback Equalizer (DFE) 10 inaccordance with the present invention will be described. The DFE 10includes a first summing node 12 having a first input (+) for receivingan input signal, a second input (−) for receiving a second feedbacksignal second_feedback, and an output. A maximum likelihood sequenceestimator (MLSE) 14 for estimating a symbol sequence has an inputconnected to the output of the first summing node 12 and receives thesignal data_in. A second summing node 16 has a first input (+) connectedto the output of the first summing node 12, a second input (−) forreceiving a first feedback signal first feed_back, and an output.

The DFE 10 also includes a signal level decoder 18 having an inputconnected to the output of the second summing node 16, and a delay line20. The signal level decoder 18 or slicer is preferably a 4-level pulseamplitude modulation system (PAMS) for determining whether the signal isat one of four levels, e.g. plus or minus 1, and plus or minus 3, aswould be readily appreciated by the skilled artisan. The delay line 20includes a first plurality of taps T₀-T₃ being connected to the outputof the signal level decoder 18, and generating respective first tapsignals based upon respective first coefficients C₀-C₃. Also, the delayline 20 includes a second plurality of taps T₅-T_(n) being connected tothe output of the MLSE, and generating respective second tap signalsbased upon respective second coefficients C₅-C_(n). Of course any numberof taps T may be used based upon the requirements of a particularsystem; however, the use of four taps T₀-T₃ connected to the output ofthe signal level decoder 18 has been found to be optimal in terms ofperformance, cost and benefit.

Furthermore, the DFE 10 has a first summing circuit 22 for summing thefirst tap signals to generate the first feedback signal first_feedback,and a second summing circuit 24 for summing the second tap signals togenerate the second feedback signal second_feedback. An error signalgenerator 26 has a first input (+) connected to the input of the signallevel decoder 18, and a second input (−) connected to the output of thesignal level decoder for adjusting the first and second coefficientsC₀-C_(n).

The MLSE 14 preferably has a partial output for outputting a partiallyestimated signal pre_data_out based upon the estimated symbol sequence.Note that the delay line 20 may further include a third tap T₄ connectedto the partial output of the MLSE 14. The third tap generates a thirdtap signal based upon a third coefficient C₄, and the first summingcircuit 22 sums the first and third tap signals to generate the firstfeedback signal first_feedback.

Also, the MLSE 14 preferably estimates the symbol sequence based uponthe M-algorithm and may receive the coefficients C₀-C₄ from the firstplurality of taps T₀-T₃ and the third tap T₄. The M algorithm isdescribed in an article by V. Joshi and D. Falconer entitled “SequenceEstimation Techniques for Digital Subscriber Loop Transmission withCrosstalk interference” in IEEE Transactions on Communications, Vol. 38,No. 9, September 1990. Additionally, the M algorithm is described in anarticle by J. Anderson and S. Mohan entitled “Sequential CodingAlgorithms: A Survey and Cost Analysis” in IEEE Transactions onCommunications, Vol. 32, No. 2, February 1984. Of course other knownalgorithms may be used in the MLSE 14; but, better performance has beenachieved thus far with the M algorithm.

As is apparent from the above description and from FIG. 1, the symboldelay line 20 has been split into two sections with the output data_outof the MLSE being fed to part of the delay line. Because symbols in thedelay line 20 are adjusted with the coefficients C₀-C_(n), and fedback(i.e. first_feedback and second_feedback) to the signal level decoder18, the output of the MLSE 14 is able to correct symbols whenever itsoutput is available. In such a symbol updating approach, errorpropagation in the delay line 20 is avoided during an error event.

Referring now to FIG. 2, a method of estimating symbol sequences of aninput signal comprising a plurality of symbols will now be described.The method begins (block 40) and includes summing the input signal andthe second feedback signal second_feedback to generate a first summedsignal at block 42. The first summed signal is combined with a firstfeedback signal first_feedback to generate a second summed signal atblock 44. The second summed signal is level decoded (block 46) via thesignal level decoder 18 to generate a decoded signal. At block 48,respective first tap signals are generated from the decoded signal basedupon the respective first coefficients C₀-C₃. Also, the first tapsignals are combined to form the first feedback signal first_feedbackwhich is used in the combining step at block 44. Second tap signals aregenerated (block 50) from the symbol output signal data-out based uponrespective second coefficients C₅-C_(n), and the second tap signals arecombined to form the second feedback signal second_feedback which isused in the combining step at block 42. At block 52, a maximumlikelihood sequence estimation is performed for estimating a symbolsequence of the first summed signal to provide the symbol output signaldata_out which fed back to the delay line 20 to generate the second tapsignals at block 50.

Also, an error signal may be generated based upon the second summedsignal and the decoded signal for adjusting the first and secondcoefficients C₀-C_(n). Furthermore, performing the maximum likelihoodsequence estimation may comprise generating the partial output signalpre_data_out. Here, a third tap signal may be generated from the partialoutput signal pre_data_out based upon a third coefficient C₄. Asdiscussed above, the partial output signal pre_data_out comprises apartially estimated signal based upon the estimated symbol sequence.Here, the sum of the first and third tap signals forms the firstfeedback signal first_feedback. Again, the maximum likelihood sequenceestimation is preferably based upon the M-algorithm.

Because the output of the MLSE 14 is fed into the delay line 20according to the proper delays of the particular algorithm (e.g. the Malgorithm), error propagation is reduced or prevented and the overallperformance of a receiver can be improved.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

That which is claimed is:
 1. A decision feedback equalizer (DFE)comprising: a first summing node having a first input for receiving aninput signal, a second input for receiving a second feedback signal, andan output; a maximum likelihood sequence estimator (MLSE) for estimatinga symbol sequence and having an input connected to the output of thefirst summing node, and having an output; a second summing node having afirst input connected to the output of the first summing node, a secondinput for receiving a first feedback signal, and an output; a signallevel decoder having an input connected to the output of the secondsumming node, and having an output; a delay line comprising a firstplurality of taps being connected to the output of the signal leveldecoder, and generating respective first tap signals based uponrespective first coefficients, and a second plurality of taps beingconnected to the output of the MLSE, and generating respective secondtap signals based upon respective second coefficients; a first summingcircuit for summing the first tap signals to generate the first feedbacksignal; a second summing circuit for summing the second tap signals togenerate the second feedback signal; and an error signal generatorhaving a first input connected to the input of the signal level decoder,and a second input connected to the output of the signal level decoderfor adjusting the first and second coefficients.
 2. The DFE according toclaim 1 wherein the MLSE has a partial output; and wherein the delayline further comprises a third tap connected to the partial output ofthe MLSE.
 3. The DFE according to claim 2 wherein the partial output ofthe MLSE outputs a partially estimated signal based upon the estimatedsymbol sequence.
 4. The DFE according to claim 2 wherein the third tapgenerates a third tap signal based upon a third coefficient; and whereinthe first summing circuit sums the first and third tap signals togenerate the first feedback signal.
 5. The DFE according to claim 4wherein the MLSE receives the first and third coefficients.
 6. The DFEaccording to claim 1 wherein the MLSE receives the first coefficients.7. The DFE according to claim 1 wherein the MLSE estimates the symbolsequence based upon the M-algorithm.
 8. The DFE according to claim 1wherein the second input of the first summing node comprises an invertedinput.
 9. The DFE according to claim 1 wherein the second input of thesecond summing node comprises an inverted input.
 10. The DFE accordingto claim 1 wherein the second input of the error signal generatorcomprises an inverted input.
 11. The DFE according to claim 1 whereinthe signal level decoder comprises a pulse amplitude modulation system(PAMS).
 12. The DFE according to claim 9 wherein the PAMS comprises a4-level PAMS.
 13. A decision feedback equalizer (DFE) comprising: afirst summing node for receiving an input signal and a second feedbacksignal; a maximum likelihood sequence estimator (MLSE) connected to anoutput of the first summing node for estimating a symbol sequence; asecond summing node connected to the output of the first summing nodeand for receiving a first feedback signal; a signal level decoderconnected to the second summing node; and a delay line comprising afirst plurality of taps being connected to the signal level decoder, andgenerating respective first tap signals based upon respective firstcoefficients, a sum of the first tap signals forming the first feedbacksignal, and a second plurality of taps being connected to the MLSE, andgenerating respective second tap signals based upon respective secondcoefficients, a sum of the second tap signals forming the secondfeedback signal.
 14. The DFE according to claim 13 further comprising anerror signal generator connected to the input and the output of thesignal level decoder for adjusting the first and second coefficients.15. The DFE according to claim 13 further comprising: a first summingcircuit for summing the first tap signals to generate the first feedbacksignal; and a second summing circuit for summing the second tap signalsto generate the second feedback signal.
 16. The DFE according to claim13 wherein the MLSE has a partial output; and wherein the delay linefurther comprises a third tap connected to the partial output of theMLSE.
 17. The DFE according to claim 16 wherein the partial output ofthe MLSE outputs a partially estimated signal based upon the estimatedsymbol sequence.
 18. The DFE according to claim 16 wherein the third tapgenerates a third tap signal based upon a third coefficient; and whereina sum of the first and third tap signals forms the first feedbacksignal.
 19. The DFE according to claim 18 wherein the MLSE receives thefirst and third coefficients.
 20. The DEE according to claim 13 whereinthe MLSE estimates the symbol sequence based upon the M-algorithm.
 21. Amethod of estimating symbol sequences of an input signal comprising aplurality of symbols, the method comprising: summing an input signal anda second feedback signal to generate a first summed signal; summing thefirst summed signal with a first feedback signal to generate a secondsummed signal; level decoding the second summed signal to generate adecoded signal; generating respective first tap signals from the decodedsignal based upon respective first coefficients; summing the first tapsignals to form the first feedback signal; generating respective secondtap signals from a symbol output signal based upon respective secondcoefficients; summing the second tap signals to form the second feedbacksignal; and performing a maximum likelihood sequence estimation forestimating a symbol sequence of the first summed signal to provide thesymbol output signal.
 22. The method according to claim 21 furthercomprising generating an error signal based upon the second summedsignal and the decoded signal for adjusting the first and secondcoefficients.
 23. The method according to claim 21 wherein performingthe maximum likelihood sequence estimation comprises generating apartial output signal; and further comprising generating a third tapsignal from the partial output signal based upon a third coefficient.24. The method according to claim 23 wherein the partial output signalcomprises a partially estimated signal based upon the estimated symbolsequence.
 25. The method according to claim 23 wherein a sum of thefirst and third tap signals forms the first feedback signal.
 26. Themethod according to claim 23 wherein the maximum likelihood sequenceestimation uses the first and third coefficients.
 27. The methodaccording to claim 21 wherein the maximum likelihood sequence estimationis based upon the M-algorithm.